1. Field of the Invention
The present invention relates to a process to reduce, by rolling, the outer diameter and the wall thickness of a generally cylindrical, hollow tube blank, and more particularly, to a tube forming process wherein an end portion of the hollow blank is reduced in size to compensate for the increase in size which occurs to the end portion of the tubing during the next rolling stage.
2. Background Information
During hot rolling of a steel tube, the diameter and the wall thickness of a hollow tube blank are reduced at a temperature of approximately 1,000 degrees C. This reduction is performed in a rolling mill by rolls that are, preferably, positioned symmetrically around the axis of the tube. An internal mandrel is introduced into the tube blank to support the tube during rolling. During rolling, material from the tube blank flows primarily in a longitudinal direction toward the unrolled portion of the tube blank. Because of the reduction forces generated during rolling and the geometry of the rolls and the tube blank, the tube blank has a tendency to expand in a direction transverse to the longitudinal axis thereof when the blank enters the reduction zone.
The degree of expansion of the tube blank is determined, in part, by the wall thickness of the tube blank. The thicker the wall, the greater the transverse expansion. When rolling the trailing end portion of thin-walled tubes, wherein the ratio of outer diameter to wall thickness is approximately 35:1, the expansion of the tube in the transverse direction and the resulting enlargement of the tube diameter is so great that the rolls slide against the tube, and the tube becomes jammed. Consequently, the rolling operation produces a residual funnel-shaped triangular end portion on the tube at the trailing end of the tube.
One way to alleviate this problem is to cut off the funnel-shaped portion of the tube before it enters the rolling mill. However, this operation reduces the efficiency of the rolling mill process. Overall funneling of the tube trailing end portion creates many problems. In particular, the presence of a funnel at the tube end portion prevents the production of thin-walled tubes having the desired wall thicknesses. Also, due to the formation of the funnel on the tube end the tube may become lodged or stuck in the rolling roll, and to remove the tube is a time-consuming and expensive disassembly process. As a result of the required disassembly process, the subsequent tubes in the line must then also be scrapped because the tubes cool in temperature to the extent that they cannot be used in subsequent process steps. The funnel also exerts great stresses on the rolls and operating parts. The stresses are known to reach a magnitude where the rolls tear and couplings and transmissions break. An additional disadvantage is that it becomes difficult to take measurements at the trailing end portion of the tubes.